Metals, ceramics and other important materials are composed of many individual single crystal grains. For homogenous composition materials, the crystal structure of all grains is identical, but their relative crystal orientation is not identical throughout the material. In fact many important engineering properties of materials are a function of the grain properties, such as grain size, boundaries, size distribution, and orientation, to list a few examples.
Single composition poly-crystalline materials typically have no contrast to identify individual grains and boundaries in conventional x-ray tomography scans based on absorption and/or phase contrast.
Electron backscatter diffraction imaging (EBSD) can be performed on the surface of polished cross-sections of materials in a scanning electron microscope to image grains and grain boundaries in two dimensions. The crystal orientation of grains is determined in EBSD. Serial sectioning with a focused ion beam milling tool and EBSD imaging can yield three dimensional (3-D) EBSD data. 3-D EBSD is a destructive measurement technique since the sample gets destroyed in the process, however.
Material evolution in the time domain as a function of external factors such as temperature cycling, stress or strain are extremely important to understand material failure and best processing conditions to yield materials with optimum properties. Since 3-D EBSD can only capture the grain map of a sample once, it is very unsatisfactory to study material evolution.
X-ray diffraction contrast tomography (x-ray DCT) is a non-destructive approach for obtaining the 3-dimensional characterization of polycrystalline microstructures. It allows the simultaneous mapping of the crystal grain shapes, grain orientation and microstructure of polycrystals that gives rise to absorption.
In the conventional x-ray DCT arrangement, the sample is illuminated with a monochromatic beam of high energy synchrotron radiation. As the sample is rotated, and grains pass through the illuminating beam, the condition for Bragg diffraction gets fulfilled by individual grains, these diffraction spots are recorded on a 2D detector placed behind the sample. The diffraction geometry is used to assign spots to the grains from which they arise, and to determine the crystallographic orientations of grains. The spots are used as projections of the grains to reconstruct the respective grain shapes. The technique has been applied to several materials science investigations, for example in the 3D characterization of grain boundary networks, and in-situ studies of inter-granular stress corrosion cracking in some stainless steels. Other materials investigated by x-ray DCT have included aluminum alloy Al 1050. Most importantly, it is now possible to perform routine 3-D grain map measurements non-destructively, which enables repetitive measurements to study time evolution.
The necessity to use synchrotron sources to perform these measurements is very limiting and a laboratory source diffraction CT system would close this gap. It is well known that synchrotrons generate x-rays with orders of magnitude higher brightness than laboratory sources, and the methods for DCT developed for the synchrotron require high beam brightness, which manifests itself in high beam collimation and monochromaticity.
Laboratory sources generally have very poor brightness compared to synchrotrons since they emit a very wide bandwidth of x-ray wavelengths in terms of Bremsstrahlung. Characteristic emission lines emitted in addition to the Bremsstrahlung background are low in intensity compared to total x-ray power emitted, and the use of a monochromator (crystal monochromator or multilayer) further reduces the intensity when trying to monochromatize the beam of a laboratory source.
Nevertheless, U.S. Patent Application Publication No. 2012/0008736A1, to Lauridsen et al., published on Jan. 12, 2012, describes an x-ray DCT system that can use a laboratory source. This system mirrors the implementation of a synchrotron DCT setup, in that it assumes the use of a focused and monochromatic x-ray beam. Additionally a scheme using non-standard detectors is described to detect the diffracted signal.